1. Field of the Invention
This invention relates to a sheet material with improved dyeability, and particularly to a microporous polyurethane sheet material which is easily dyed with various types of dyes and is excellent in color fastness.
2. Description of the Prior Art
Polyurethane resins generally have some affinity to a certain type of dyes (for example metal complex dyes) and they have a certain degree of dyeability for themselves. However, when utilized as artificial leathers or synthetic leathers, their dyeability is not fully satisfactory. Moreover, their dyed products have serious defects in that they discolor by water or light, or show migration of dyes.
To remove such defects, dyeing processes by azo-dyes have been proposed wherein a large amount of an alkali hydroxide is present in the grounder solution. However, since a large amount of an alkali is used in these processes, they are unsuitable in the case of dyeing leather substitutes such as artificial leathers and synthetic leathers, in view of their stability to hydrolysis.
Also, a process has been proposed wherein, upon the production of polyurethane resin, a triamine, such as N-methylamino-bis-propylamine, is used as chain extender together with hydrazine or a substituted hydrazine, and another process has been proposed wherein a low-molecular weight polyol having a tertiary or quaternary nitrogen atom in the molecule is used. However, these processes have defects such that a reaction to form a three-dimensional structure (gellation) is liable to occur or the resulting resin has poor physical properties. There is also a process wherein a vinyl monomer is graft-polymerized to a polyurethane resin which has been prepared beforehand. However, these processes has a disadvantage in that the process takes a long time and is complicated.